Split source_management.go along its CRUD seams (closes #274)

Pure code movement. Every declaration of
internal/handlers/source_management.go moves unchanged into one of:
webhook_list.go, webhook_create.go, webhook_detail.go, webhook_edit.go
and webhook_delete.go for the webhook pages; event_log.go for the event
log; entrypoint.go for the entrypoint handlers; target_create.go,
target_delete.go and target_toggle.go for the target handlers; and
shared.go for the helpers several of them use. Only each file's package
line and imports are new. The README and a middleware comment that
named the removed file now name the new ones.

Model: opus-5-5

internal/handlers/target_delete.go

@@ -0,0 +1,31 @@
+package handlers
+
+import (
+	"net/http"
+
+	"sneak.berlin/go/webhooker/internal/database"
+)
+
+// HandleTargetDelete handles deleting a target. A deleted
+// database target's archive writer is evicted and its handle
+// closed; the archive file is left on disk.
+func (h *Handlers) HandleTargetDelete() http.HandlerFunc {
+	return h.deleteChildResource(
+		"targetID", &database.Target{},
+		"failed to delete target",
+		h.evictTargetArchiveWriter,
+		targetDeleted,
+	)
+}
+
+// evictTargetArchiveWriter is evictArchiveWriter for one deleted
+// target, and leaves its archive file on disk for the same reason.
+// A target that is not a database target has no writer, and
+// evicting it does nothing.
+func (h *Handlers) evictTargetArchiveWriter(targetID string) {
+	if h.archives == nil {
+		return
+	}
+
+	h.archives.EvictTarget(targetID)
+}